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Shareholders' equity (Details) - USD ($)
 12 Months Ended 33 Months Ended 84 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2017
Mar. 31, 2015
Dec. 31, 2011
Dec. 31, 2008
Class of Stock [Line Items]                
Common stock, shares authorized (shares) 195,000,000 195,000,000   195,000,000 195,000,000      
Common stock, par value (in dollars per share) $ 0.01 $ 0.01   $ 0.01 $ 0.01      
Common stock, shares issued (shares) 94,731,298 94,437,418   94,731,298 94,731,298      
Common stock, shares outstanding (shares) 87,544,777 87,250,897   87,544,777 87,544,777      
Preferred stock, shares authorized (shares) 5,000,000 5,000,000   5,000,000 5,000,000      
Preferred stock, par value (in dollars per share)   $ 0.01            
Preferred stock, shares issued (shares)               160,000
Conversion of convertible preferred stock (shares) 100     100 100      
Liquidation preference (in dollars per share) $ 0.01     $ 0.01 $ 0.01      
Authorized repurchase amount $ 130,000,000     $ 130,000,000 $ 130,000,000   $ 60,000,000  
Additional repurchase amount           $ 70,000,000    
Treasury stock acquired, aggregate purchase price     $ 36,352,000   86,276,000      
Remaining authorized repurchase amount $ 43,724,000     $ 43,724,000 $ 43,724,000      
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Repurchase of common stock (shares)       0 7,186,521      
Series A Convertible Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock, par value (in dollars per share) $ 0.01     $ 0.01 $ 0.01      
Preferred stock, shares outstanding (shares) 74,364 75,625   74,364 74,364      
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Beginning balance (shares) 75,625 76,539 78,141          
Repurchase of common stock (shares) 0 0 0          
Conversion of convertible preferred stock (shares) (1,261) (914) (1,602)          
Issuance for share-based compensation plans (shares) 0 0 0          
Ending balance (shares) 74,364 75,625 76,539 74,364 74,364      
Treasury [Member]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Beginning balance (shares) 7,186,521 7,186,521 4,786,521          
Repurchase of common stock (shares) 0 0 2,400,000          
Conversion of convertible preferred stock (shares) 0 0 0          
Issuance for share-based compensation plans (shares) 0 0 0          
Ending balance (shares) 7,186,521 7,186,521 7,186,521 7,186,521 7,186,521      
Common stock [Member]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Beginning balance (shares) 87,250,897 87,038,050 89,064,582          
Repurchase of common stock (shares) 0 0 2,400,000          
Conversion of convertible preferred stock (shares) 126,098 91,362 160,162          
Issuance for share-based compensation plans (shares) 167,782 121,485 213,306          
Ending balance (shares) 87,544,777 87,250,897 87,038,050 87,544,777 87,544,777